Modern industrial manufacturing technology is producing ever increasing mechanization in fabrication, and inventory and quality control, frequently utilizing electronic data processors and computers. Mass production of electronic data processors, computers, software and supporting services is now widespread, but connection of this technology with a multiplicity of mechanical functions in manufacturing and everyday life has necessitated much subsidiary inventive activity. One area of major concern is the transformation of the movement of material into an electronics signal. Such material may vary from material-in-process or tools in a factory to bulk commodities, goods, or merchandise in a warehouse or retail store. The present invention is broadly directed at all types of electronic surveillance.
It is, however, designed with circuitry that is analogous to that previously known in the field of pilferage control. In fact, while the present invention is not particularly limited to pilferage control, it is especially well suited to that application. In that application it is established to secure specifically constructed telltale tags to merchandise which is likely to be pilfered, and it is known to electronically monitor the exits of stores and ware houses, etc., where such merchandise is dispensed to ascertain that the tags are deactivated or detached in the manner provided for authorized removal of the merchandise. In the past various methods and apparatus along these lines have been employed, as recited in U.S. Pat. Nos. 3,895,368; 3,711,848; and 3,707,711, and in Applicant's copending applications, Ser. No. 689,336, filed May 24, 1976, now U.S. Letters Pat. No. 4,087,802, Ser. No. 883,059 filed Mar. 3, 1978, but many of these known methods and apparatus have limitations on their reliability, tolerance and sensitivity. Some are susceptible to false triggering by metallic structures coincidentally manifesting similar properties to the special tags. In some, proximity of the human body to the apparatus tends to mask the effect of the equipment and to interfere with reliable operation.
The limitation on the method and apparatus disclosed in the above patents are such that their respective systems have proved incapable of discerning with a high level of reliability whether a tag has been moved into a zone being monitored, i.e. a control zone, or is merely in proximity to it. This causes too many false signals when there is no telltale element actually in a control zone, and virtually eliminates their applicability to industrial electronic surveillance usage. Moreover, the invention disclosed in U.S. Pat. No. 3,895,368 has the additional limitation that the frequencies selected for use therein were limited by an attempt to avoid frequencies that would be very susceptible to false triggering, and thus there is little or no choice of frequencies for the plurality of remotely distinguishable control zones likely to be needed for industrial applications. Furthermore, that system is subject to over-range difficulties, particularly if the LF or electrostatic signal is strengthened, as needed if employed with multiple control zones within a larger surveillance area, which the present invention contemplates.